Certain analytical instruments require that a liquid sample be nebulized (vaporized), i.e., converted to an aerosol (a fine spray or mist), and then broken down to atoms in preparation for analysis. Thus, a nebulizer (typically pneumatically assisted) is often part of the sample introduction system of an optical emission spectrometer (OES, also termed an atomic emission spectrometer or AES) or a mass spectrometer (MS). The aerosol from the nebulizer is directed into a plasma plume generated by a plasma source, which is often configured as a flow-through torch. The plasma is typically inductively coupled plasma (ICP) or microwave induced plasma (MIP). Exposure to plasma breaks the sample molecules down to atoms.
In the plasma, the sample atoms repeatedly lose electrons (are ionized) and recombine with electrons. During this process, the atoms emit electromagnetic radiation (light) at wavelengths characteristic of their elemental identities. In an OES, this light is collected and focused by optics and directed to an analyzer, which may include, for example, a diffraction grating. The analyzer spectrally resolves the light into its component wavelengths, enabling the intensity of the light at each wavelength (respective abundances of the wavelengths) to be measured by an optical detector. The OES system then presents the data so acquired as a spectrum of atomic emission lines. The intensity of each line is indicative of the concentration (abundance) of the corresponding element of the sample.
In an MS, ions of the sample atoms are extracted from the plasma source and directed as an ion beam into a mass analyzer. The mass analyzer applies a time-varying electrical field, or a combination of electrical and magnetic fields, to spectrally resolve different types of ions on the basis of their mass-to-charge (m/z) ratios, enabling an ion detector to count each type of ion of a given m/z ratio. The MS system then presents the data so acquired as a spectrum of mass (m/z ratio) peaks (respective abundances of the m/z ratios). The intensity of each peak is indicative of the concentration (abundance) of the corresponding element of the sample.
Generally, the structures and operations of various types of analytical nebulizers, plasma sources, OES instruments, and MS instruments are known to persons skilled in the art, and accordingly are only briefly described herein as necessary for understanding the subject matter being disclosed.
In addition to the analytes (the sample atoms or ions for which data is sought), the liquid sample may contain a high concentration of dissolved salts (e.g., metal salts, such as in a sample of seawater) or total dissolved solids (TDS). During the process of converting the liquid sample to liquid drops in a gas-assisted nebulizer, the salt or dissolved solids may precipitate out of the solution and accumulate at any orifice of the nebulizer through which the sample material flows, such as the exit of the nebulizer. Over time, more precipitates form and eventually clog the exit or other orifice and the nebulizer stops working, consequently disrupting the operation of the associated analytical system. Upon detecting the clog, the operator of the system must shut the system down and remove the nebulizer for cleaning.
As an example, a gas-assisted nebulizer often has a concentric configuration in which the bulk liquid sample flows through a central tube (or capillary) and the nebulizing gas (e.g., argon, nitrogen, etc.) flows through an outer tube surrounding the central tube. A meniscus is formed at the outlet of the central tube from which the bulk liquid sample exits. When the meniscus becomes large enough, the surface of the meniscus is exposed to the gas flowing by the outlet of the central tube. A liquid drop is pulled from the meniscus when the force of the gas is strong enough and the surface of the meniscus large enough to overcome the surface tension of the liquid. Conventional nebulizers such as this type have features that present locations where the liquid is stagnant and not exposed to the gas flowing by the central tube outlet. Over time, the concentration of the salt or dissolved solid in the liquid can increase to the point where the salt or dissolved solids will precipitate out of solution. One this process begins, the precipitate formation increases and eventually a clog will occur.
One way to minimize clogging is to dilute the liquid sample and thereby lower the salt or solid concentration as described, for example, in U.S. Pat. No. 7,671,329, the content of which is incorporated by reference herein in its entirety. However, dilution may lower the measurement signals acquired and reduce the sensitivity of the spectrometer or other analytical instrument.
Therefore, there is a need for an analytical nebulizer, sample atomizer, and associated system in which clogging is minimized in a manner that does not require dilution of a liquid sample.